1. Field of the Invention
The present invention relates to wellbore apparatus, such as Logging While Drilling (LWD) and wireline logging apparatus, and, in particular, to replaceable and slide-on antennas for such wellbore apparatus. It is also applicable to electromagnetic telemetry used in Measurement While Drilling (MWD) and LWD operations.
2. Description of Related Art
Wellbore tools used downhole include Directional Drilling Systems such as MWD and LWD systems. FIG. 1 shows a conventional LWD system comprising a bottom hole assembly that includes a telemetry section 10, an LWD collar 12 also known as a “drill collar” 12, a positive displacement motor (PDM) 14, a bent sub 16, and a drill bit 18. The drill collar 12 typically includes a plurality of antennas 20 mounted thereon, the antennas 20 generally include at least one transmitter antenna 20a and at least one receiver antenna 20b. The transmitter(s) and receiver(s) each comprise loop antennas consisting of a plurality of wire turns forming a coil. These transmitter 20a and receiver 20b antennas are placed around the drill collar 12. The LWD system is used to measure subsurface properties such as formation resistivity, formation porosity, formation density, or the natural formation radioactivity. The LWD apparatus is similar to a standard wireline logging suite, except that it is incorporated into a drill collar of the LWD system.
An LWD resistivity tool measures the resistivity of the earth formation by emitting, from the transmitter, electromagnetic energy that propagates through the formation. The receiver receives the electromagnetic energy propagating in the formation and, responsive thereto, the phase and the amplitude of the electromagnetic energy are measured. When two receivers are employed, the phase shift and attenuation of the electromagnetic energy are measured between the receivers and the resistivity of the formation is deduced from the aforementioned phase shift and attenuation. In common practice, most LWD resistivity tools use electromagnetic energy in the frequency range of hundreds of kilohertz to a few megahertz. A typical distance between a transmitter and receiver is generally less than one meter because of the high rate of attenuation of high frequency electromagnetic waves in many subsurface formations.
Electromagnetic energy is also used for short-range communication between downhole systems when it is difficult to establish a direct-wired connection. For example, sensors can be placed in a small sensor sub located near the drill bit; where the sensors measure the borehole inclination and/or formation properties. If a positive displacement mud motor is used, then running an electrical wire from the sensor sub, through the mud motor, and to the MWD system is difficult. An electromagnetic antenna can be placed on the sensor sub to transmit data to the MWD system by transmitting electromagnetic energy. A similar electromagnetic antenna placed in the MWD system receives the transmitted energy and subsequently transmits the data to the surface via mud pulse telemetry. An apparatus using such a technique is described in U.S. Pat. No. 6,057,784. These electromagnetic antennas can be loop antennas similar to those used for measuring formation resistivity. However, typical frequencies used in downhole electromagnetic telemetry systems tend to be in the range of a few kilohertz to tens of kilohertz. The lower frequencies are required to transmit electromagnetic energy distances of tens of meters between the downhole tools. The higher frequency electromagnetic energy used in most LWD resistivity tools might be too attenuated in low resistivity formations. Hence, details of the low frequency antennas (such as the number of turns) can be different from the high frequency antennas.
Conventional manufacturing processes for placing the loop antennas on downhole tools, such as the drill collar 12 of the LWD system, involve disposing one or more grooves in the drill collar. A layer of fiberglass epoxy is disposed in each of the grooves, the fiberglass epoxy layer is cured, and then the layer of fiberglass epoxy is machined. A coil is then placed or grafted over the top of the machined and cured fiberglass epoxy layer in each of the grooves to produce one or more multi-turn coils in each of the grooves. A second fiberglass epoxy layer is placed over each of the multi-turn coils and a layer of rubber is placed over the second fiberglass epoxy layer. These techniques are described in U.S. Pat. No. 4,949,045 (assigned to the present assignee). A shield is typically mounted over the rubber layer, the shield including a plurality of slots to allow for the passage of electromagnetic energy.
This manufacturing process is relatively expensive and time consuming. There are basically six steps in the process: (1) put the first layer of fiberglass epoxy down and cure that layer, (2) machine that layer, (3) wind the coil, (4) put another layer of fiberglass epoxy down, cure and machine that layer, (5) put a layer of rubber over the last aforementioned layer, and (6) cure that layer, and machine the rubber. Therefore, if a loop antenna of a receiver or transmitter, which is mounted on a downhole tool, is damaged in the field, the entire tool (that is, the entire drill collar of the LWD tool) must be sent back to a central repair facility. This requires fairly large pieces of equipment and several cycles, taking many weeks to complete the process in a repair operation. In short, there is a slow turn-around process in connection with the manufacture and/or repair of the loop antennas that comprise the transmitters and receivers of typical downhole tools.
FIGS. 2a-2d show conventional antenna configurations on a drill collar 12. The antennas are wrapped around the collar in a recess 22 and oriented in different directions. In FIG. 2a, an axial coil 26 is formed of one or more loops of wire where each loop of wire lies in a plane essentially perpendicular to the axis of the drill collar. Each loop is essentially circular and centered around the drill collar 12. The orientation of the coil is indicated by the dashed arrow 201, which is perpendicular to the plane of the coil and coincident with the axis of the drill collar. The well known “right hand rule” of electromagnetic theory for determining the direction of the magnetic field for a coil carrying a current “I” can be used to determine the direction of the arrow (i.e. up or down in the FIG.). The purpose of such an axial antenna acting as a transmitter is to produce a magnetic field parallel to the axis of the drill collar. Similarly, such an axial antenna acting as a receiver will detect a magnetic field parallel to the axis of the drill collar.
In FIG. 2b, the antenna coil 62 is wrapped around the recess 22, but the plane of an individual loop is not perpendicular to the axis of the drill collar. The orientation of this coil is perpendicular to the plane of the loop, and illustrated by the arrow 202. This will be referred to as a diagonal coil, since the orientation 202 of the coil 62 is diagonal with respect to the axis of the drill collar. In this case, each loop of the coil has an approximately elliptical shape centered on the axis of the drill collar. The purpose of such a diagonal antenna coil acting as a transmitter is to produce a magnetic field diagonal to the axis of the drill collar, with the magnetic field direction indicated by the dashed arrow 202. Similarly, such a diagonal antenna acting as a receiver will detect a magnetic field diagonal to the axis of the drill collar, in the direction indicated by the dashed arrow 202.
In FIGS. 2c and 2d, two transverse antenna coils 210 are placed on opposite sides in the recess 22 of the drill collar 12 (FIG. 2d is a view rotated by 90° from FIG. 2c). Each transverse antenna 210 consists of one or more loops of wire where the plane of a loop is essentially parallel to the drill collar axis and intersects the drill collar axis. The orientation of a transverse coil is illustrated by the dashed arrows 203. If the two transverse loops are wired in series such that the direction of current is the same in both coils (i.e. counter-clockwise in FIG. 2c), then the two transverse coils have the same orientation. The purpose of such transverse antennas acting as a transmitter is to produce a magnetic field transverse or perpendicular to the axis of the drill collar, with the magnetic field direction indicated by the dashed arrow 203. Similarly, such transverse antennas acting as a receiver will detect a magnetic field transverse to the axis of the drill collar, in the direction indicated by the dashed arrow 203.
Since conventional loop antennas include a plurality of coil forms wrapped around the tool, it would be advantageous to be able to manufacture the coil forms separately and independently from the tool. With this capability one need only replace the damaged or faulty antenna in the field instead of sending the entire drill collar back to the central repair facility. This capability is especially important since such equipment is used around the world, and shipping heavy equipment into or out of some remote locations or countries is very difficult, time consuming, and costly.
Thus a need remains for improved techniques for constructing antennas separately and independently from downhole tools and for deploying such antennas on the tools.